Frequency independent unidirectional antenna



Nov. 28, 1961 D. E. ISBELL 3,011,168

FREQUENCY INDEI ENDENT UNIDIRECTIONAL ANTENNA Filed Oct. 20, 1958 2Sheets-Sheet 1 Fig. 2

INVENTOR. Dwight E. lsbe/l Merriam, Larch 8 Smifl;

A TTOfi/VEYS Nov. 28, 1961 Filed Oct. 20, 1958 D. E. ISBELL FREQUENCYINDEPENDENT UNIDIRECTIONAL ANTENNA 2 Sheets-Sheet 2 Fig. 6

INVENTOR.

Dwig/rf E. lsbel/ Merriam, Larch 8 Smith AT T OR/VE Y5 United StatesPatent 3,011,168 FREQUENCY INDEPENDENT UNTDIRECTIONAL ANTENNA Dwight E.Isbell, Urbana, Ill., assignor to The University of Illinois Foundation,a non-profit corporation of Illinois 1 Filed Oct. 20, N58, Ser. No.768,297 3 Claims. (Cl. 343-908) This invention relates to antennas andmore particularly it, relates to antennas-having unidirectionalradiation patterns that are essentially independent of frequency overwide bandwidths.

It is 'known that an antenna whose geometry is described completelybyangles, such as an infinite biconical antenna, wouldmake an idealbroadband radiator sinceits operation is theoretically completelyindepend ent of frequency; The theoretical performance of the infinitebiconicalantenna 'is not achieved in practice, however, since'such anantenna must be of finite length, and the end effect, -i.e., the effectof finite rather than infinite length, leads to radiationcharacteristics showing considerable variation-with frequency. I

A planar antenna closely related to the biconical antenna, the bow-tie,antenna, is likewise theoretically frequency-independent when infinitein size. The endefiect of an actual bow-tie antenna, however, limits therange of frequencies for which the radiation pattern is essentiallyconstant to a bandwidthof 2 or 3 to 1.

It is known that bow-tie antennas can be modified in a way which reducesthe end-effec to such an extent as to permit bandwidths of to 1' or moreto be achieved with structures of practical size. In general, thisresult is achieved by introducing periodic discontinuities along themarginal edges of the bow-tie antenna, the geometry of thediscontinuities being such that all dimensions involved are directlyproportional to the distance from the feed point of the antenna, i.e.,the vertex or the narrowest portion of the bow-tie. Planar antennas ofthis type have bidirectional radiation patterns extendingperpendicularly to the .plane of the antenna.

It has now been found that the above-describedmodified planar bowtieantennas can be further modified to exhibit unidirectional radiationpatterns while maintain-ing the broadbandwidths which such antennaspossess. The unidirectional radiation patterns are achieved byrotatingone half of the.bow -tie about an axis passing through thevertexin the plane of the antenna perpendicular to the line whichbisects both halves of the antenna, so that the antenna is no longerplanar. In the 3,011,168 Patented Nov. 28, 1961 and OD) contain aplurality of similar slots and teeth bounded by straight lines (e.g., E0and A0) and arcs The radii r r bear a constant relationship to radii R Rwhich is defined by =a constant less than 1.

a= =a constant greater than T and less than 1.

The value of dteeth.

It can be seen that, because of the geometry of the antenna as definedabove, the dimensions of any tooth (or slot) are directly proportionalto its distance from the feed point.

Referring to FIGURE 2, a side view of the antenna of FIGURE 1, it can beseen that an'angle 1,9 (less than 180) exists between the elements ofthe antenna. This configuration is produced from the similar planarantenna by rotating one of the elements about an axis lying in the planeof the antenna, said axis passing through the feed point perpendicularto the line bisecting the vertex angles of the elements. I

Infinite antenna structures of the type ofthe invention have theproperty that, when energized at the vertex, the fields at a frequency fwill be repeated periodically at all other frequencies given by m whereu may take on any integral value. The parameter 1- determines .what maybe considered the bandwidth of a period of operation. That is,

thus determines the widthof the slots and f1 where f and f are twofrequencies exactly one period apart (f f The shape of theantenna'structures of preferred embodiments of the invention, the angleformed at the vertex by'the planes of the'halves of the antenna FIGURES3, 4, 5 and 6 are typical radiation patterns 1 .for the antenna ofFIGURE 1.

FIGURE 7 is a sketch identifying theco-ordinate system used in FIGURES3, 4, 5 and 6. l

As shown in FIGURE 1, the antennas of the present invention comprise twoidentical substantially V-shaped,

electrically conducting elemen-ts each of which is partiallydefined byintersecting straight lines (e.g., A0 and I, CD) the apexes of saidelements almost meeting (i. e., no electrical connection) at -point ;O(the'feed point of the antenna). The marginal edges of each element(e.g., A0

1 its complement are fitted together theycover the whole:

the invention is such that the variation of the radiation pattern andimpedance is small over one period, and because of the periodicallyrepeating nature of the fields, the same will be true for all periods,the result being an extremely broadband antenna. For the finitestructures of the invention it has been found that the fields along theantennas decay very rapidly after passing a point where a resonantdiscontinuity, such as a tooth one quarter wavelengthlong, exists. Thisdecay of the field causes the end effect of the antennas of'theinvention to be small, so that Wide bandwidths arereadily obtained withstructures of finite size. I a

The lower and upper-limits of the frequency band in which the radiationpatterns are independent of frequency are determined by the longest andthe shortest teeth, respectively, in'the antenna. The low frequencylimit is that for which the longest teeth are wavelength long. Likewise,the high frequency limit is that for which the shortest teeth arewavelength long. It can therefore be seen that the bandwidth of theantenna can be adjusted as desired by making the shortest and thelongest teeth in the antenna correspond to wavelength of the desiredfrequency limits.

It .is known that for planar antennas, including the rime versions ofthe antennas of this invention (i.e., 0:180"), any antennawhichh'a'stlie same shape as its complement has [a constant irripedancejwhich is' inde .pendent of frequency.complem' ent of a planarantenna isdefined as the portionof theplane which is' not covered by the originalantennaz'when an antenna and plane without overlapping. It is preferredthat the antennas of the invention be self-complementary in the plane inorder to minimize the variation of impedance with frequency. This can beaccomplished in the antenna of FIGURE 1, for example, by making the sumof angle a and angle [3 equal to 90.

Although the particular embodiment of FIGURE 1 (i.e., in which the slotsand-teeth are defined by circular arcs) is preferred, the invention isnot limited thereto. Other types of discontinuities can be used alongthe edges of the antenna, such as trapezoids, triangles, etc, providedthat the dimensions of all such discontinuities are proportional to thedistances from the feed point. Regardless of the actual configurationused for the discontinuities, it is preferred that all discontinuitiesin a given antenna have similar shapes.

It can be seen that the antenna of FIGURE 1 possesses no axis ofsymmetry but'is symmetrical about the feed point only when the elementsare coplanar, i.e., when =180. Symmetry about the feed point is definedas a configuration such that for every point in the antenna falling on astraight line passing through the feed point, there exists acorresponding point on the same line'at an equal distance on the otherside of the feed point. Although antennas made in accordance with theinvention which possess one or more axes of symmetry exhibit somebroadband characteristics, the bandwidths thereof are in generalinferior to those of antennas which are symmetrical about the feed-pointonly, and which are therefore preferred.

In the preferred embodiment the same arcs which define the teeth andslots at one marginal edge of one of the elements of the antenna alsodefine the teeth and slots on the other marginal edge of the sameelement. However, in order to avoid symmetry about any axis, the slotsand teeth on the edges are arranged such that each tooth has acorresponding slot and each slot has a corresponding tooth on theopposite edge of the element at the same distance from the feed point,the corresponding slot or tooth being a mirror image of the tooth orslot to which it corresponds.

In order to demonstrate the performance of the antennas of theinvention, a number of antennas of the type shown in FIGURE 1 wereconstructed of light gauge copper sheet, approximately ,5 inch thick.The periodic discontinuities were in the form of teeth defined by arcsof circles connected to a sector-shaped central conducting strip. Inorder to maintain an equi-complementary condition, the sum of angles atand ;8 was made equal to 90. A small area near the center of the antennawas left as solid conductor since continuation of the teeth in thatdirection requires an infinite number of teeth of'zero width in thelimit. For all models tested, the ratio a, as defined above, was takenequal to the square root of the ratio 1-, also defined above, providinga ratio of tooth to slot width which is the same for all rows of teeth.This, however, is not a necessary condition'since can assume any valuegreater than 1 but less than 1.

Example I An antenna of the type of FIGURE 1 was constructed with thefollowing values afiixed to the variables:

R =9 inches The antenna was fed across the vertex with a coaxial linehaving its outerconductor bonded to one half of the antenna and itsinner conductor extended across the vertex and connectedto the otherhalf. The radiation patterns 'for this antenna were measured at afrequency of 1400 me. of various values of angle #1. The effect ofrotating thev elements of the planar antenna out of the plane about anaxis passing through the vertex was to cause it to radiate more in onedirection than in the other. The front to back ratio was found toincrease as the angle 0 was reduced, the change being gradual down toabout with a rapid increase as the angle was further reduced. Theradiation patterns for four values of t are shown in FIGURES 3, 4, 5 and6. In FIGURES 3-6 the vertical axis at 12 oclock represents 0:0", andthe radiation patterns are orientated on this basis. The designation =90represents a constant condition applicable to each figure, indicatingthat the radiation patterns are those taken in the =90 plane (i.e., theYZ plane as seen in FIGURE 7). The E, variations represent thehorizontally (i.e., YZ plane) polarized component of the beam while theE, variations represent the vertically (i.e., perpendicular to the YZplane) polarized component of the beam. It can be seen that excellentunidirectional performance was obtained as a result of inclining theelements of the antenna as described.

The electric field produced by an antenna of the invention lies in thehorizontal or YZ plane when the antennais positioned as shown in FIGURE7. It is, therefore, evident that the polarization of these antennasdiffers from that of a dipole having its arms folded to form a V, whichhas its electric field in the plane determined by the elements of thefolded dipole.

It was further found that the radiation patterns for this antenna wereindependent of frequency over a bandwidth of more than 10 to 1. Inaddition, the bandwidth was found to be independent of angle 1/ for therange from 30 to 100.

Several additional models identical to that of Example 1 except forvalues ofr were tested. Essentially no change was noted in'the radiationpatterns as 'r was reduced until a value of 0.5 was reached, at whichpoint beamwidth and pattern variations were noted for the smaller valuesof 4/. Although the patterns remained essentially intact, they wereinferior in frequency independence to those obtained for the highervalues of T, which are accordingly preferred.

Example II An antenna similar to that of Example 1 was constructedhaving the following parameter values:

The radiation patterns for-this antenna were found to be similar tothose for the antenna of Example 1.

The input impedance of the non-planar antennas of the invention exhibitsomewhat more variation with frequency than, is found in the similarplanar antennas, which variation increases with decreasing values of it.The impedance characteristics of the antennas tested were found to beessentially the same for'all models, showing little variation with thevalues of 'r and tooth angle a. The mean resistance level, however, wasfound to decrease with decreasing 1p, ranging from 165 ohms at =180 toapproximately 70 ohms at =30.

The two halves of the antennas of the invention are fed at the verticesthereof either with a balanced two wire line or with a coaxial linehaving the outer conductor bonded to the vertex of one half of theantenna and the inner conductor attached to the vertex of the clearnessof understanding only, and no unnecessarylirni tations should beunderstood therefrom, as modifications will be obvious to those skilledin the art.

What is claimed is: r

1. An antenna comprising a pair of opposed electrically conductingsubstantially V-shaped elements lying in different planes, the apexes ofsaid elements being immediately adjacent each other and togetherconstituting the feed point of said antenna, said elements forming anangle at said feed point of about 30 to about 100", each of saidelements having an outline partially defined by a respective pair ofintersecting straight lines, the edges of each of said elements definedby said straight lines containing a plurality of alternating slots andteeth, the dimensions of each of said slots and teeth being proportionalto its distance from said feed point.

2. A non-planar antenna comprising a pair of opposed planar,electrically conducting, substantially V-shaped elements lying indifferent planes, the apexes of said elements being adjacent each otherand together constituting the feed point of said antenna, the planes ofsaid elements forming an angle at the feed point of about 30 to about100, each of said elements comprising an imperforate central sectorshaped portion and slotted sector-shaped portions of equal angular widthon either side of said central portion, said slotted portions containinga plurality of alternating slots and teeth, the dimensions of 6 each ofsaid slots and teeth being proportional to its distance from said feedpoint.

3. An antenna as described in claim 2 which is equal to its complement.

References Cited in the file of this patent UNITED STATES PATENTS2,192,532 Katzin Mar. 5, 1940 2,480,154 Masters Aug. 30, 1949 2,615,005White Oct. 21, 1952 FOREIGN PATENTS 937,360 Germany Jan. 5, 19561,100,801 France Sept. 26, 1955 OTHER REFERENCES Development of Fan TypeTV Antenna," Radio and Television News, May 1950, pp. 66, 67, 132.

DuHamel and Isbell: Broadband Logarithmetically Periodic AntennaStructures," March 1957 IRE National

